[ RadSafe ] Survey Instrument Question

[Dimiter Popoff]

I would say the manufacturer is correct - given that the calibration
applies to each unit's individual geometry. Obviously the smaller one
will see less.

If the calibration applies to some other geometry common to the two
units they should see the same - and the manufacturer would be wrong.

Dimiter

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>From: Bob Westerdale <Bob.Westerdale at ametek.com>
>To: "radsafe at health.phys.iit.edu" <radsafe at health.phys.iit.edu>
>Date: Fri, 29 Jul 2016 19:41:22 +0000
>Subject: [ RadSafe ] Survey Instrument Question
>
>Hi All!
>       We recently had two survey meters calibrated ( same manufacturer, different
>models)-  both are Geiger instruments, one has a 45mm dia tube, other has a very
>small ( ie about 10 mm dia) tube, both with comparable thickness  Mica windows.
>The Energy dependence curves are fairly similar and within 10%  of each other at
>50 keV,  and both are fundamentally intended to be used at less than 100 mR/hr.
>We survey Analytical XRF equipment,   Max.  Tube voltage is 50 kV.  
>
>     Our tech was using both meters during a routine survey, and noticed the smaller
>device was reading only about 1/3 of what the larger one was seeing. We're below 
>1 mR/hr so pulse pileup or detector time constant issues should not be a problem.
>( well within the published countrate specs) The tech asked me to investigate,  so
>we contacted the Manufacturer ( who also did the calibration)  who responded that:
>" , the size of the window and overall size of the tube, is what gives each unit
>a different reading"  
>
>He maintained that both units were working correctly.
>
> I am aware of the measurement constraints and correction factors necessary when the
>radiation being measured illuminates only a portion of the active area of the detector,
>we're dealing here with a fairly uniform radiation field that would fully cover both
>of the detector's windows.      I always thought that a calibration should include
>review ( and tweaking if needed) of the  CPS / mR  factor that would be used to provide
>a reasonably accurate ( maybe +/- 15%) determination of the doserate regardless of
>the detector's dimensions. I'm also aware of the limitations of using a Geiger counter
>for low energy measurements;  I use an ionization chamber  ( with appropriate corrections)
>when an accurate doserate must be determined.  
>
>Is the comment from the manufacturer valid?
>
>Thanks in advance!
>Bob Westerdale
>EDAX, Inc.
>RSO
>